A radiator which is a conventional automobile aluminum heat exchanger is shown in FIGS. 1(a) and (b). In the automobile aluminum heat exchanger shown in FIG. 1, a core 4 is assembled by placing fins 2 on tubes 1, in which a coolant flows, and attaching header plates 3 to the ends of the tubes 1. After brazing the core 4, the radiator is obtained by attaching resin tanks 5A and 5B to the header plates 3 through a backing 6. As the coolant of the radiator, a weak alkaline aqueous solution containing antifreeze containing an anticorrosive, namely a so-called long life coolant (LLC), and the like are used.
Regarding the materials of these components, a plate of an Al—Mn-based alloy containing Zn having a thickness of about 0.1 mm is used for the fins 2. The material used for the tubes 1 is a clad aluminum alloy material having a thickness of about 0.2 to 0.4 mm obtained by cladding an Al—Zn-based alloy as a sacrificial anode material on the coolant side of a core material of an Al—Mn-based alloy to prevent corrosion perforation due to the coolant and cladding an Al—Si-based alloy as a brazing filler metal on the side open to air. A clad aluminum alloy material having a thickness of about 1.0 to 1.3 mm and having a similar structure to that of the tubes 1 is used for the header plates 3.
The clad aluminum alloy materials used for the tubes 1 and the header plates 3 are exposed to atmosphere at about 600° C. during braze heating. Therefore, Zn added to the sacrificial anode material forms a diffusion layer of Zn in the core material. It is known that a perforation is not formed for a long time because the presence of the diffusion layer of Zn causes the corrosion caused in the sacrificial anode material in an acidic environment to spread in the lateral direction also after reaching the core material.
Although an LLC is a weak alkaline liquid containing an anticorrosive as described above, water of poor quality having no anticorrosive effect, such as well water and river water, is also sometimes used as the coolant. The water of poor quality is sometimes acidic, and in this case, corrosion is prevented by the sacrificial anode effect of the sacrificial anode material as described above.
It is known that an LLC, which is a weak alkaline liquid containing an anticorrosive, deteriorates during the use and turns into a strong alkali, thereby forming a corrosion pit in the tube material. The sacrificial anode material does not have the sacrificial anode effect on corrosion in an alkaline environment, and the corrosion advances isotropically. As a result, a perforation is formed at an early stage. Thus, various designs for preventing corrosion in an alkaline environment have been investigated.
PTL 1 proposes a clad aluminum alloy material obtained by cladding a brazing filler metal on one surface of a core material and cladding a sacrificial anode material on the other surface, wherein the sacrificial anode material is composed of an aluminum alloy which contains Ni and in which an Al—Ni-based intermetallic compound is dispersed.
In the clad aluminum alloy material, in the part of the sacrificial anode material surface where the Al—Ni-based intermetallic compound is present, deposition of aluminum hydroxide, which is the component of coating, is prevented and the coating formation is prevented. According to PTL 1, the prevention of the coating formation results in many small defects of coating, and the corrosion is dispersed. It is described that such dispersed small coating defects prevent the corrosion from advancing in the depth direction more readily than localized coating defects, and the generation of a perforation can be prevented also in an alkaline environment.
PTL 2 describes that addition of Si with Ni to the sacrificial anode material results in the formation of an Al—Ni—Si-based intermetallic compound, which is dispersed more finely and densely than an Al—Ni-based intermetallic compound, and thus better corrosion resistance in an alkaline environment is obtained.
However, the recent trend of heat exchangers is towards reduction in weight and size, and thus the thicknesses of the materials are desired to be reduced. Reducing the thicknesses of the materials means that the thickness of the sacrificial anode material is also reduced. In the two PTLs, only the sacrificial anode material has the anticorrosive effect in an alkaline environment, and once a part of a corrosion pit reaches the core material, the corrosion advances rapidly from the point. Therefore, the corrosion resistance in an alkaline environment decreases remarkably when the thickness of the sacrificial anode material is reduced.
PTLs 3 and 4 propose materials having a structure capable of exhibiting corrosion resistance even when a part of a corrosion pit reaches the core material in an alkaline environment, and the structure is obtained by adding Ni also to the core material and evenly dispersing an Al—Ni-based intermetallic compound in the core material.
However, in the materials, the core material corrodes at the same rate as the corrosion rate of the sacrificial anode material, and thus the reduction in thickness cannot be stopped at the sacrificial anode material. As a result, the materials have problems of a decrease in the strength as a clad material and a decrease in the sacrificial anode effect when the liquid turns acidic through the replacement of the coolant with water of poor quality. Also, because a large amount of a coarse Al—Ni-based intermetallic compound is contained in the core material, there are problems of a decrease in the crystal grain size after braze heating and a harmful effect on the brazing property.